Gas barrier films and packaging materials using the same are already well known. Of these, although aluminum foil is known to have the most excellent oxygen gas barrier properties, it cannot be used as it is but for special uses, for its pinhole resistance is too weak. Therefore, the aluminum foil is mostly used as an intermediate layer of a laminated film. The gas barrier properties of the laminated film are far excellent, but the film is opaque and therefore an object contained therein cannot be observed therethrough, hence difficulty in judging whether the film has surely been heat-sealed or not.
As the oxygen gas barrier film, films of polyvinylidene chlorides or vinylidene chloride copolymers (hereinafter referred to simply as PVDC) and PVDC-coated films are well known. Particularly, PVDC-coated films are known as laminated films with excellent barrier properties against oxygen gases and water vapor. PVDC hardly absorbs moisture and can exhibit its good gas barrier properties even under conditions of high humidity. Therefore, a variety of base films, on which a layer of PVDC is coated, can be employed regardless of their moisture permeability. Examples of the base film include biaxially stretched films of polypropylene, nylon, or of polyethylene terephthalate, and cellophane. Taking advantage of their gas barrier properties, these laminated films are utilized for wrapping or packing of a variety of foods, regardless of their being dry or moisture-laden. These packaging materials, after being used, are disposed as non-industrial, domestic wastes from homes. These wastes when incinerated give off toxic and hazardous gases and, what is worse, are causes of highly carcinogenic chlorine-containing organic compounds generated upon incineration at low temperatures. For such reasons, transition to the use of other materials has strongly been desired. However, the reality is that alternatives with performance and cost-performance equivalent to those of PVDC are not yet found.
For example, as the oxygen-gas barrier film, a polyvinyl alcohol (PVA)-series film is also well known. Only if the degree of its moisture absorption is low does the PVA film show extraordinarily excellent gas barrier properties against oxygen. However, the PVA film is originally high in moisture absorption, and its gas barrier properties against oxygen are immediately deteriorated upon exposure to an environment with a relative humidity of 70% or higher. Therefore, the PVA film has been considered to be lacking in practicality. For improving the moisture absorption of PVA, there has been proposed a copolymerization of PVA with ethylene into ethylene.vinyl alcohol copolymer (EVOH), a polycondensation with alkoxysilane by the sol-gel method [Japanese Patent Application Laid-Open No. 345841/1992 (JP-A-4-345841)], and a modification of part of the alcohol of PVA to make it water-resistant. However, none of the methods provides resins with satisfactory performance.
Moreover, although there have been suggested that a polyamide film obtained by reacting an aliphatic dicarboxylic acid with 4,4′-methylene bis(phenylisocyanate) shows excellent gas barrier properties and thermal resistance [Japanese Patent Application Laid-Open No. 252631/1989 (JP-A-1-252631)] and that a polyallyl alcohol-containing aqueous dispersion for gas barrier coating and a multilayered structure having a layer made therefrom exhibit excellent gas barrier properties and transparency [Japanese Patent Application Laid-Open No. 140072/1998 (JP-A-10-140072)], they are still unsatisfactory in their gas barrier properties under conditions of high humidity and water resistance.
There have been produced films with high oxygen-gas barrier properties, which are made by depositing an inorganic oxide such as silicon oxide [Japanese Patent Publication No. 12953/1978 (JP-B-53-12953)] and aluminium oxide [Japanese Patent Application Laid-Open No. 179935/1987 (JP-A-62-179935)] on a film. Since such inorganic oxide-made film is fabricated through a physical or chemical deposition process, the base film thereof itself is required to be deposition-durable, and therefore only to a limited variety of base film materials can be adopted. In addition, as these films are made of inorganic oxides, they are inferior in flexibility and tend to crack in the course of their secondary processing, possibly resulting in the degradation of the gas barrier properties.